Aromatic π-Electron Density Modulation in Isostructural Metal-Organic Frameworks Enables Efficient Natural Gas Upgrading

Abstract

Efficient separation of C₂H₆ and C₃H₈ from CH₄ remains a significant challenge due to their similar molecular dimensions and nonpolar nature. This study resolves this bottleneck by realizing π-electron modulation of aromatic pore surfaces via strategic ligand functionalization. Three isostructural metal-organic frameworks with dia topology, namely Ni(3-fpba)₂, Ni(3-mpba)₂, and Ni(3-npba)₂, were synthesized with substituents of varying electronic characters. The progressive enhancement of electron-donating ability systematically enriches the π-electron density, which strengthens C–H···π interactions with alkane molecules in a size-discriminative manner. Notably, the amino-functionalized Ni(3-npba)₂ exhibits a C₂H₆ uptake of 80.5 cm³·g^–1 at 273 K and 10 kPa, representing a 96% enhancement over its fluorinated counterpart while maintaining negligible CH₄ adsorption. This performance leads to IAST selectivities of 30.4 for C₂H₆/CH₄ and 398 for C₃H₈/CH₄, surpassing most benchmark materials. Dynamic breakthrough experiments and GCMC simulations collectively reveal that enhanced π-electron density intensifies host-guest charge redistribution. These findings establish aromatic π-electron density as a tunable descriptor for the rational design of natural gas purification materials.

Keywords

Metal–organic framework / Natural gas purification / π-Electron modulation / C–H···π interaction / Isostructural design / Gas adsorption / Dynamic separation / Crystal engineering

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